1. Field of the Invention
The present invention relates to an image processing device for efficiently compressing image information, and more particularly to an image processing device for performing high-efficiency coding of a continuous-tone character image.
2. Description of the Related Art
An image is a kind of information that can be grasped intuitively and entirely, and communication using the image is widely spread as means having a high function of information transfer. However, because of the fact that image information is generally very large in information content, a cost and a processing time for transfer of image information become large if the image information in the original form is transmitted through a communication network to a remote site, or is accumulated into various electronic media. To cope with this problem, it is necessary to use a high-efficiency coding method for efficiently compressing the image information and transmitting or accumulating the compressed information.
As such a high-efficiency coding method known are a predicative coding method, block coding method (BTC: Block Truncation Coding), orthogonal transform coding method, vector quantization method, and entropy coding method.
However, such various high-efficiency coding methods conventionally proposed have their merits and demerits depending on the property of a subject document image, and are insufficient in coding a document image containing characters, line drawings, halftone image, etc. in combination.
To solve this problem, the present inventors have proposed in Japanese Patent Laid-open Publication No. 4-144485 an image processing device which can hold a high image quality, highly compress image information, and code/decode the image information in a short processing time.
In this image processing device previously proposed by the present inventors, an input image is divided into a character area and a halftone area. The character area is further divided into a plurality of blocks, and each block is coded by a coding method adaptable to the statistical property of the character area. On the other hand, the halftone area is also coded by a coding method adaptable to the statistical property of the halftone area.